1. Field of the Invention
The present invention relates generally to X-Ray Compton scatter imaging.
2. Description of the Related Art
Through the advancement of computing systems, conventional medical and industrial imaging systems provide an increased level of information available to the researcher. X-ray computed tomography can be used for medical imaging and industrial imaging methods employing tomography created by computer processing. A computed tomography (CT) scan can produce a large amount of data that can be manipulated, through a process known as “windowing”, in order to demonstrate various bodily structures based on their ability to block the X-ray beam. These CT methods, also called computed axial tomography scan (CAT scan), use digital geometry processing to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images, typically taken around a single axis of rotation.
Medical researchers have used CT to supplement X-ray and ultrasonic imaging. Because of its ability to detect various types of bodily tissue, the use of CT in preventative measures has increased. For example, CT is increasingly being used to screen for various types of diseases such as cancer and conditions such as heart disease. There are several advantages that CT has over traditional 2D medical radiography. First, CT can eliminate the superimposition of images of structures outside the area of interest. Second, because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished. Finally, data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task. This is referred to as multiplanar reformatted imaging.
Although the availability and usage of CT has increased dramatically over the last two decades, the increased use of X-rays has caused some concern. Although the data and results are not conclusive, it is estimated that 0.4% of current cancers in the United States are due to CTs. CT scans involve the use of 10 to 100 times more ionizing radiation than typical X-rays. Estimated lifetime cancer mortality risks attributable to the radiation exposure from a CT in a 1-year-old are 0.18% (abdominal) and 0.07% (head)—an order of magnitude higher than for adults—although those figures still represent a small increase in cancer mortality over the background rate.
In the diagnostic energy range (20-140 keV), photons interact with matters via three fundamental mechanisms: photoelectric absorption, coherent scattering and Compton scattering. To maximize dose efficiency, i.e. amount of information per dose, an ideal x-ray diagnostic imaging system should utilize all the useful information from the interactions between x-ray photons and the object, including photoelectric absorption, coherent scattering and Compton scattering. Conventional x-ray CT imaging obtains the total linear attenuation coefficients from the three mechanisms mentioned above, and therefore does not provide an effective amount of patient information per dose.
Many techniques have been proposed to acquire images from each of the three mechanisms. For example, photoelectric response has been used to track the nano-particles injected into human bodies for cancer therapy. Phase-contrast imaging measures coherent scatter signals and significantly increases image contrasts as compared to conventional CT imaging. Compton scatter imaging exhibits many merits over conventional CT imaging as well. It provides accurate electron density distributions, which lays a solid foundation for precise radiation dose calculation in both diagnostic and therapeutic energy ranges.
Compton scattering is a type of inelastic scattering that X-rays and gamma rays (both photons with different energy ranges) undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy (increase in wavelength) of an X-ray or gamma ray photon, called the Compton Effect. Part of the energy of the X/gamma ray is transferred to a scattering electron, which recoils and is ejected from its atom (which becomes ionized), and the rest of the energy is taken by the scattered, “degraded” photon. The amount the wavelength changes by is called the Compton shift.
Compton scatter images can also be combined with the conventional CT images for contrast enhancement and material decomposition. Systems specialized for Compton scatter imaging have been designed since the early days of CT. Based on the targeting methods of scatter sources, the data acquisition modes of these systems can be divided into two categories. The first group targets the scatter sources point-by-point by moving a diverging-hole collimator and the detector together. This relative movement can severely degrade data acquisition efficiency. The second type of method traces the multiple sources of measured scatter photons using their energy information. These methods require an energy-sensitive detector, which is expensive using the current manufacturing technologies and is not available on commercial volumetric CT (VCT) systems. Thus, conventional Compton imaging systems are either inefficient on dose and imaging time or require an energy-selective detector.